A woodland near Woodhall Spa remains cordoned off after two people were hospitalised having unearthed canisters containing mustard gas.
Roughton Woods also known as OSLA Plantation, has been cordoned off after the discovery of canisters found by people bottle digging on Sunday, October 1.
The canisters were taken to Porton Down, the Defence Science and Technology Laboratory, to identify what they contained.
Police now understand the contents to be mustard gas and it is believed the canisters have been in situ since when the site was an operational RAF base.
No-one has been seriously injured; two people who found the canisters were taken to hospital and received treatment for burns and minor respiratory problems on Sunday, they returned home the same day.
If anyone has recently recovered any bottles or other containers from this area they are asked to contact police on 101.
Officers are working with other emergency services, military and health partners to ensure the continued safety of anyone who uses the woods and to establish if any further canisters are present.
Police would ask people to avoid the area for the time being.
The area, formally known as RAF Woodhall Spa, opened in February 1942 as a satellite of RAF Coningsby and closed in the mid 1960s.